Image display apparatuses which display images by scanning a laser beam are expected to be developed since they have a wide color reproduction range and are capable of high contrast display by a simple construction. Particularly, they are suitable when miniaturization is required such as in the case of mobile projectors and HMDs.
For example, various methods including a method using a pixel display device such as a liquid crystal element or an organic EL as an image display and a method directly forming an image on the retinas of eyes by two-dimensionally scanning a laser beam have been proposed for image display apparatuses such as HMDs (head-mounted displays) which display images while being mounted on the head of a user.
Such image display apparatuses are required to be small in size and light in weight as a whole in order to reduce a mounting burden on a user to enable prolonged use. Further, if an image display apparatus is constructed in the same design as generally used spectacles, a user can act while constantly wearing the image display apparatus like normal spectacles.
However, as image quality and a view angle increase, an eyepiece optical system using a display, a prism for introducing light emitted from the display to the eyes and a half mirror becomes larger with a method using a pixel display device. Thus, miniaturization and weight saving are difficult.
Further, a large eyepiece optical system as described above is structured to cover the eyes and is shaped like a goggle or a helmet rather than spectacles. It is difficult to expect natural wear comfort and it is difficult to realize a general spectacle-type image display apparatus.
On the other hand, a retinal scanning image display apparatus using a laser scanning method has an advantage of being able to be made very small using a small-size MEMS (Micro-Electro-Mechanical-System) mirror device.
In an image display apparatus required to have a very small construction like the spectacle-type HMD described above, a resonant mirror with which a large displacement can be obtained by a small drive force is suitable. Further, a two-axis resonant MEMS mirror capable of two-axis scanning by one chip is most suitable.
Normally, upon scanning a laser beam by a scanning mirror, it is preferable to perform scanning at a high speed in a horizontal direction and perform scanning in accordance with a frame rate of moving images to be displayed, at 60 Hz for instance, in a vertical direction. If the scanning mirror is designed to resonate at such a low frequency, the rigidity of a spring structure for supporting the mirror is reduced and the mirror becomes very weak against disturbances such as vibration in many cases.
Given this situation, there has been proposed a method for resonantly driving a scanning mirror in two axes at a higher frequency, that is, a method for displaying by Lissajous scanning.
For example, in a disclosed relationship of frequency and phase when images are displayed by Lissajous scanning using two-axis resonant scanning, two-axis resonant frequencies are set at a ratio of relatively prime integers to make the frame rate of images and a round frequency of scanning integral multiples (see, for example, patent documents 1 and 2).
On the other hand, the resonant frequencies of the scanning mirror are determined by the mass of a movable portion and a spring constant of a supporting structure. It is difficult to produce the scanning mirror while accurately designating the resonant frequencies due to dimension and thickness variations in manufacturing.
Thus, it has been proposed to make the resonant frequencies themselves variable such as by providing a heating unit near a twist beam of a vibration mirror for heating (see, for example, patent document 3).
Alternatively, a method has been proposed which equalizes the numbers of scanning lines by temporarily stopping scanning by a vertical scanner during a period until a horizontal scanning start signal corresponding to a vertical synchronization signal of a video signal in a second frame is input after scanning by a horizontal scanner in a first frame is finished, thereby realizing synchronization with the frame rate (see, for example, patent document 4).
However, the conventional constructions as described above have the following problems.
In the scanning display apparatus using the two-axis resonant mirror, since it is difficult to synchronize a cycle of scanning to image the entire screen and the frame rate of moving images, the frames are switched during screen scanning. Thus, frame boundaries appear particularly in images with quick and rushed movements and discontinuity is viewed at a specific position of the screen in some cases.
If the mechanism for making the resonant frequencies variable is provided, the device becomes complicated and additional power is necessary, which undermines characteristics of small size and power saving.
Resonantly driven devices are incapable of making special movements for synchronization such as temporary stop of slower scanning.    Patent Document 1: Japanese Translation of PCT International Application No. 2005-526289    Patent Document 2: Japanese Unexamined Patent Publication No. 2007-093644    Patent Document 3: Japanese Patent No. 4172627    Patent Document 4: Japanese Unexamined Patent Publication No. 2008-065310